Contact materials based on silver with carbon, particulary with graphite, are widely used as protective switches in low-voltage power technology because they afford a high safety against a welding of the contacts. In most cases the contact material contains the carbon as a powder. Because silver and carbon are not soluble one in the other in a solid or liquid state, such materials can be made only by powder metallurgy. It is known to mix silver powder and graphit powder with each other and to make components by compacting said mixture, sintering and re-compacting, or to make blocks in that the powder mixture is isostatically cold-pressed, followed by sintering and shaping by extruding so that the graphite particles are oriented in the extruding direction to form fiberlike agglomerates (see A. Keil et al., "Electrische Kontakte und ihre Werkstoffe" Springer-Verlag (1984), page 195, and the company publication entitled "Graphor Kontaktwerkstoffe aus Silber-Graphit" which has been published by the applicant with the imprint 4/90):Said agglomerates are often described in the literature simply as graphite fibers. The formation of such a fibrous structure is particularly pronounced in AgC materials which are made by a repeated extruding of sheath wires filled wtih graphite powder (see K. Auller and D. Stockel, German periodical "Metall" 36 (1982), Page 743).
On the other hand, whereas the silver-graphite materials have a very high resistance to welding, they have also an unsatisfactory resistance to erosion and this is a disadvantage. An increasing graphite content will increase not only the resistance to welding but also the erosion. For this reason the requirements for a high resistance to welding and for a low erosion mutually preclude each other in silver-graphite contact materials.
In the contact material the graphite powder results in a hardening which is similar to dispersion hardening so that the material has a low ductility and the subsequent shaping of the contact elements is highly expensive.
Attempts have been made to increase the resistance of contact materials to erosion by an incorporation of fibers of a high-melting material (U.S. Pat. No. 3,254,189, U.S. Pat. No. 4,699,763, Published German Application 20 57 618). Published German Application 20 57 616 discloses the use of continuous filaments of carbon or graphite or of a "wool" of carbon filaments. Said filaments are impregnated with molten silver or copper, which may contain an additive consisting of 0.5 to 4% by weight flake graphite for improving the lubricating properties of sliding contacts. Because graphite is not wetted by copper, silver and their alloys, it is necessary to add a carbide-forming agent, such as titanium. But it has been found in practice that even with the use of such a wetting agent it is extremely difficult to make corresponding materials by an impregnation of a bundle of fibers or a wool of carbon filaments. Said difficulties may be avoided by the process which is described in U.S. Pat. No. 4,699,763 and in which silver powder, graphite fibers and various additives are mixed to form a slip and in a plurality of powder-metallurgical operations are processed to make contact platelets. Such materials contain carbon in the form of true carbon fibers, as is disclosed in Published German Application 20 57 618, or of graphite fibers, as is disclosed in U.S. Pat. No. 4,699,763, and their testing under conditions of use has shown that they have a distinctly higher resistance to erosion than a composite material made with graphite powder but that the resistance to welding is drastically decreased. For this reason an appreciable use of the materials made in accordance with U.S. Pat. No. 4,699,761 has not become known.
It is an object of the invention to provide a contact material which is based on silver with carbon or graphite and is superior as regards erosion and workability to the known contact materials which are based on silver with grahpite powder but as regards resistance to welding does not have the severe disadvantages of a contact material based on silver and carbon fibers.
That object is accomplished by a material having the features resited in claim 1. Desirable further features of the invention are subject matters of the dependent claims.
The contact material in accordance with the invention distinguishes in that it contains carbon in the form of pieces of fibers in combination with a content in the form of a powder. It has surprisingly been found that in the material in accordance with the invention the values for the erosion and for the resistance to welding are much more favorable than would result from the application of the rule of mixtures to the selected ratios of carbon fibers to carbon powder. The combined use of carbon fibers and carbon powder produces a result which could not have been predicted from the known effects of the individual components.